An example of a prior art power supply apparatus for use with an AC arc welder is shown in FIG. 1. The power supply apparatus of FIG. 1 has power supply input terminals 2a, 2b and 2c, to which an AC supply, e.g. a three-phase commercial AC supply (not shown) is connected. The power supply input terminals 2a–2c are connected to an input-side rectifying circuit 4. The input-side rectifying circuit 4 rectifies a first AC voltage, e.g. a three-phase commercial AC voltage from the three-phase commercial AC supply and develops a rectified voltage. The rectified voltage is smoothed and changed to a DC voltage by a smoothing capacitor 6. The resultant DC voltage is applied to DC-to-high-frequency converter means, e.g. a high-frequency inverter 8 and converted into a high-frequency voltage. The high-frequency inverter 8 has a plurality of semiconductor switching devices, e.g. IGBTs, bipolar transistors or FETs although not shown. An inverter control circuit 9 controls the conduction period of the semiconductor switching devices. The high-frequency voltage from the inverter 8 is applied to a primary winding 10P of a transformer 10 having a secondary winding 10S in which a boosted high-frequency voltage is induced. The boosted high-frequency voltage is rectified by diodes 12a, 12b, 12c and 12d of an output-side rectifying circuit 12. The rectified voltage is developed between a positive output terminal 12P and a negative output terminal 12N. The rectified voltage is smoothed by smoothing reactors 14a and 14b, to develop a main DC voltage. Thus, the input-side rectifying circuit 4, the smoothing capacitor 6, the inverter 8, the transformer 10, the output-side rectifying circuit 12, and the smoothing reactors 14a and 14b form a DC power supply.
The main DC voltage from the DC power supply is applied to a DC-to-AC converter 16, which produces from the main DC voltage, a second AC voltage. The second AC voltage is applied across a load, or between, for example, a workpiece 18 and a torch 20 via output terminals 17A and 17B of the power supply apparatus. The DC-to-AC converter 16 includes a semiconductor switching device, e.g. an IGBT 22a, connected to the positive output terminal 12P of the output-side rectifying circuit 12 through the smoothing reactor 14a. Specifically, the collector of the IGBT 22a is connected through the smoothing reactor 14a to the positive output terminal 12P, and the emitter of the IGBT 22a is connected to the workpiece 18. Similarly, the DC-to-AC converter 16 has a semiconductor switching device, e.g. an IGBT 22b, connected through the smoothing reactor 14b to the negative output terminal 12N of the output-side rectifying circuit 12. Specifically, the emitter of the IGBT 22b is connected through the smoothing reactor 14b to the negative output terminal 12N, and the collector of the IGBT 22b is connected to the workpiece 18. The transformer 10 has an intermediate tap 10T on the secondary winding 10S. The intermediate tap 10T is connected to the torch 20. Anti-parallel diodes 24a and 24b are connected across the respective emitter-collector conduction paths of the IGBTs 22a and 22b. 
A control signal at a predetermined frequency (e.g. at several tens of hertz (Hz) to several hundred hertz (Hz)) is supplied to the gate of each of the IGBTs 22a and 22b from a DC-to-AC conversion control circuit 26, and the IGBTs 22a and 22b are alternately rendered conductive. When, for example, the IGBT 22a is conductive, a current of the positive polarity flows from the positive output terminal 12P through the smoothing reactor 14a, the IGBT 22a, the workpiece 18 and the torch 20 to the intermediate tap 10T, during a period Tpe shown in FIG. 2A. Conversely, when the IGBT 22b is conductive, a current of the negative polarity flows from the intermediate tap 10T through the torch 20, the workpiece 18, the IGBT 22b and the smoothing reactor 14b to the negative output terminal 12N during a period Tne. In this manner, an AC arc is generated between the workpiece 18 and the torch 20 for welding.
In this case, if the welding current is smaller than, for example, 50 A, the energy stored in the smoothing reactors 14a and 14b is insufficient even when the welding current has a rectangular waveform as shown in FIG. 2A, and, therefore, an arc, sometimes, cannot be re-generated between the torch 20 and the workpiece 18 at the transition of the welding current from the positive polarity to the negative polarity. In this specification, terms “re-generate”, “re-generation” and “re-generated” used in association with an arc or arcing are meant to express re-generation of an arc, which is once extinguished at the transition of a welding current from positive to negative, after the welding current changes to negative.
In order to help the re-generation of arcing, another voltage supply 28 providing an arc re-generating voltage to be superposed is used. The superposition voltage supply 28 includes a voltage-boosting transformer 30 having its primary winding 30P is connected between the power supply input terminals 2a and 2b for receiving therefrom an AC voltage, so that a boosted AC voltage is induced in a secondary winding 30S of the transformer 30. The boosted AC voltage is rectified by a rectifier 32 and smoothed by a combination of a resistor 34 and a capacitor 36 to form a DC voltage. The transformer 30, the rectifier 32, the resistor 34 and the capacitor 36 form an auxiliary voltage supply 28.
The peak value of the DC voltage from the combination of the resistor 34 and the capacitor 36 is larger than the negative peak value of the voltage applied between the workpiece 18 and the torch 20 during the negative polarity period of the AC welding voltage. The DC voltage is applied between the torch 20 and the workpiece 18 through a series combination of a semiconductor switching device, e.g. a bipolar transistor 18, and a current limiting resistor 40, and the IGBT 22b, in such a manner that the workpiece 18 is negative and the torch 20 is positive. More specifically, the DC voltage is applied when the IGBT 22b is conductive, so that the workpiece 18 is negative and the torch 20 is positive. The application of the DC voltage is carried out by rendering the bipolar transistor 38 conductive. The conducting period of the bipolar transistor 38 is controlled by a control signal applied to the base of the bipolar transistor 38 from a superposition voltage control circuit 42, which starts providing the control signal to the bipolar transistor 38 in synchronization with the generation, by the DC-to-AC conversion control circuit 26, of the control signal to render the IGBT 22b. The control signal is applied to the bipolar transistor 38 for a period shorter than the period during which the welding voltage is negative. A diode 44 is connected in parallel with the series combination of the current limiting resistor 40 and the bipolar transistor 38. The diode 44 becomes conductive when the input voltage to the DC-to-AC converter 16, i.e. the voltage between the intermediate tap 10T and the negative output terminal 12N, transiently increases, to thereby cause the capacitor 36 to absorb the transient voltage.
When the polarity changes from positive to negative, the transistor 38 is rendered conductive, and a DC voltage as shown in FIG. 2B is supplied from the auxiliary voltage supply 28, which facilitates the re-generation of an arc. In this case, since the capacitor 36 and the resistor 40 form an integration circuit, the DC voltage discharged from the capacitor 36 rises gently.
The above-described power supply apparatus requires a voltage higher than, for example, 200 V to be applied between the output terminals 17A and 17B in a state in which the workpiece 18 and the torch 20 are not connected to the output terminals 17A and 17B, in order for an arc to be re-generated between the workpiece 18 and the torch 20, or requires a voltage even higher than 250 V for stable arc re-generation. Accordingly, if the voltage supplied at the input terminals 2a–2c becomes low due to voltage variations, a voltage sufficient for stable arc re-generation sometimes cannot be applied between the workpiece 18 and the torch 20.
An object of the present invention is to provide a power supply apparatus which can reliably re-generate an arc even when a small welding current changes from positive to negative.